Prevention of screeching combustion in jet engines



' Filed Feb. 9, '1956 March 8, 1960 H. u. WISNIOWSKI 2,927,423

PREVENTION OF SCREECHING COMBUSTION m JET sncmss 3 Sheets-Sheet l March 8, 1960 u. w|s |ow 2,927,423

PREVENTION OF SCREECHING COMBUSTION IN JET ENGINES Filed Fab. 9, 1956 a Sheets-Sheet 2 March 8, 1960 Filed Feb. 9. 1956 H. u. WISNIOWSKI i pgug PREVENTION OF SCREECHING COMBUSTION IN JET ENGINES Sheets-Sheet 3 dam! PREVENTION OF SCREECHING COMBUSTION IN JET ENGINES Henryk U. Wisniowski, Cardinal Heights, Ontario, Canada Application February 9, 1956, Serial No. 564,574

1 Claim. (Cl. 60-35.6)

This invention relates to improvements in jet engines and is directed towards the avoidance of the phenomenon known as screeching combustion (also known as screaming or howling). This condition may be encountered in any kind of jet engine, but is particularly prevalent in the jetpipe of turbojet engines operating with reheat (or afterburning) Screeching in the jetpipe of a turbojet engine is a noise which is produced by high frequency oscillations of the gas column in the jetpipe. The oscillations that give rise to screeching take place in a direction transverse to the longitudinal axis of the jetpipe and they often exert a destructive eflfect on various components of the engine as well as being accompanied by an increase in the rate of heat transfer. Screeching combustion has been known to lead to the destruction of the metal of jetpipes and flameholders, to crack open welded seams and bosses, to break igniters and thermocouples and to loosen bolts. Screeching combustion is normally encountered when the rate of fiow of reheat fuel is high and this phenomenon may thus represent a limiting factor in the quantity of reheat fuel that can be employed, being thus effective to limit the augmentation of engine thrust that can be obtained by means of reheat.

Prior investigations have already shown that the vibrations in question are high frequency gas oscillations extending transversely across the jetpipe and it has been proposed to suppress screeching by placing longitudinal ribs in the jetpipe. It has also been suggested to use perforated or corrugated inner liners. Such measures are, however, cumbersome and uncertain as to operation, and it is the primary object of the present invention to provide a simpler method for inhibiting screeching combustion under reheat conditions in the jetpipe.

It has now been discovered that the gas oscillations which cause screeching originate on the flameholder (sometimes known as the stabilizer). It follows that the formation of such oscillations might well be prevented if the conditions in the neighbourhood of the flameholder conducive to the formation of such oscillations could be modified.

Furthermore, it has now been discovered that the oscillations originate at the downstream edge of the flameholder and the present invention consists of the introduction of streams of fluid into the path of the combustion gases flowing rearwardly from the downstream edge of the flameholder, so as to disturb the normal flow conditions of gases in the jetpipe at this point. The fluid of these streams may be a separate fluid such as fuel, or water, or some introduced gas such as air, or it may be a portion of the combustion gases themselves.

Experiments have confirmed that the introduction of such streams of fluid produces very satisfactory'results in preventing screeching.

In the case where a separate fluid is used and this fluid is fuel, it is preferred to direct it in a manner such that the streams will initially be inclined both to the direction of travel of the combustion gases and to the transverse direction in which the oscillations take place, al

though it has been found that the boundaries between which the fluid may be directed and still provide improvement in avoidance of screeching, are not critical. The stream of fuel may, with advantage, be projected rearwardly and outwardly of the downstream edge of the flameholder, and may be such as approximately to form a continuation of the surface of the flameholder. However, when water is used as the fluid, the best results are obtained by' directing the streams towards the inside of the flameholder. 1

It should be appreciated that, when fuel is used for this purpose, it only forms a small proportion of the total additional fuel used for reheat purposes. The main reheat fuel is always introduced upstream of the flame holder. Moreover, it is not intended that this fuel, or other fluid when air or water is used, should necessarily be continuously projected from the downstream edge of the flameholder. Normally this operation will only require to be initiated temporarily when screeching is encountered, either by manual control, or automatically by suitable means sensitive to the screeching condition.

When the streams of fluid are formed from a portion of the combustion gases themselves, it is convenient to produce such streams by means of structural alteration to the flameholder. For this purpose a vane may be disposed adjacent said flarneholder to form therewith a pas sageway converging in the downstream direction and terminating in a narrow slot adjacent the downstream edge of the flameholder. The combustion gases that enter this passageway will be accelerated and will issue from the slot at a higher velocity than the remainder of the combustion gases flowing past and around the flameholder.

The accompanying drawings are providedto illustrate the two basic ways in which the invention may be carried into practice.

Figure 1 shows diagrammatically a portion ofthe general layout of a turbojet engine;

Figure 2 is a detailed side elevational view of a cone type of flameholder embodying means according to the invention;

Figure 3 is an elevation view of Figure 2" as seen from the right, i.e. from the downstream side;

Figure 4 is a view similar to Figure 2 showing a modified form of flameholder;

Figure 5 is a view similar to Figure 3 but showing the flameholder of Figure 4; v

Figure 6 shows in side elevation the invention applied to an annular type of flameholder;

Figure 7 is an elevation view of the flameholder as seen from the right of Figure 6, i.e. from the downstream side;

Figure 8 is a view similar to Figure 6 of a modified form of annular flameholder;

Figure 9 is a view similar to Figure 7 but showing the flameholder of Figure 8; I

Figure 10 is a side elevation view of a still further modified form of annular flameholder; I

Figure ll is a view of the flameholder of Figure 10 seen from the downstream direction;

Figure 12 shows diagrammatically a cross-secti'dn of a fragment of a V gutter type of flameholder "modified in accordance with the second aspect of the invention wherein a portion of the existing combustion gases are employed to form the introduced streams of fluid;

Figure 13 shows an elevation view of'a complete flameholder of the V-gutter type modified as in Figure 12 and seen from the upstream side; I t

Figure 14 is a similar upstream elevation view of 0 V-ring type of flameholder modified in the manner of Patented Mar. 8, 1960 ameness" he -present invention; and

Figure ,17 .is an elevationrview seenirom the down- ,streamgside of another such eompositeuiorm' ofrflame- .;.holder. a

Figure. .l illustrates diagrammatically,v the combustion .c hamber:-.ands other parts ofsthe jet'engine. In the ,1 t-casepof a turbojetrengiue, ion instance, air is blown into the main chamber lihfrom La compressor 8 .travelling in the direction of the arrows IL theMmainI fuelrbeing .;-injected into the comhustion chamben. 10.5mm .a pipe :1, 12,,Tfhe hoe combustiontgasestthus produced pass through theqturbine 9 and into thejetpipe 13e, The reheatfuel is injected into the jetpipenfroma pipe l4 situated somewhat upstream from a fiameholderlfi; shown .in Figure 1 as ,eing of the cone type; Ihisflameholden-issupported in sthe jetpiper13 by a'strut 16.,rFiguregl thusillustrates very diagrammatically: the basic elements of a conventional Zturbojet engine-operating with reheat. Eiguresi2t-and 3; showthe. fiameholder 15 modified accordingtot the: first aspect of the invention. tSuch modifis rcationr comprises. the arrangement of a ghollow tube 17 extending r. around, the sinner ...downstream, edge ofic the lfiameholder 15;..vThis tube 17 isconnected to taa snpply ipe l 8 .and .is formedwith a series of uholes 19 at its t earward surface, suchholes igsbeing arrangedeto f orm nozzles,that-will direct a. closely spacedseries ofliiuid prays whichtcombineIto rorm conical spray 20 that approximams .toa continuation of theCOl1lCa1 ShaP of the fia meholder .15 itself. That is to say themean angle .,-between-rthecconical spray 29 and;theslongitudinalgaxis ofthejetpipe lfi wouldibe of; the order of"'45.

a ig-htlymiodified form oft this-constructionis seen is iguresA.Yandi wherea tube 17'.. is .situated on; the outside of the downstream edge of a flameholder 157.1,Ihe e tube 1:7 is similarly, providedswith a series of holes 19' adapted to form a conical rearwardly projecting: spray a 20'. Thevfiuid.is,supplied by a. pipe 18' as before.

liiguresfi to 9tillustratejsimilar constructions applied to the annular type of flameholder (sometimes-called a hy ringnflarneholder or. stabilizer) lnFiguresad and 7 a pair of perforated tubes .2 1.and.-22 is provided, thetube 21 extendingaround the innenperiphery. of an; annular flameholder 23 and the tube 22 extendingaround the. outer .s. .zsperipher-y of such Jlameholder; so that, at any :section through the flameholder, a spray 2.4,produced by the @geinner tube 21 and aspray 25;.produced by thettube 22 will" define rearward..con tinuatious oflthe tapering surfaces of the inner.. and-.outer-.sides of th'eflameholder. The tu e 21- i fe r masupply pipe 26 and the, tube 22 is'suppl'ied from a supply pipe 27. In practice, often only valone .of the tubes 21 or 22r1will licensed for eliminating screeching: They may both benused inccases of. very intense rscr -eechin g.

' Figures 8 and 9 are identical except thatntuhes 211 and 22's are positioned on. the, oute f 'dqwustr eam edgeaof an annular fiameholder 23'; analogously with theiarrangement f 'ES APZ aS PPIl-Q p p d f d e-(tu :"and' 22' respectively,..and sprays lofifluid Zfland 251 are Eigures l0; and l'1- illustrate an annularatype Lofr flame- 1older.;-1nodified; for use, withwwatert l rthisg case. the Aubes, .here designated 21?; and 22i'warepositioned on the inne'nidownstreamnedge of an .annulanfiameholder 2 generally 'similarjtq; Figure 6, but theperforations rmfltesn ys estr n e t d ect their/alter Wardly of the Mildred-ann l rin ;defi eni henfiameholder itself These-zsprays of "waters are" indieatgd at- 24:22 and 2sfj=analogouslymwith t-he' spray s shown m the previous example. In the case whereswat'er. is the disttnrbanoe;aorflovmpast: the sflameho'lden that sordanceewith a modified form -of. the second aspect of s H inhibits screeching is then '4 caused mainly by steam that is formed within the interior of the flameholder.

In accordance with the second aspect of the present invention, the streams of fluid may be derived from the combustion gases themselves. To do this, it is necessary to increase the velocity of a portion of these gases and then project the high velocityv gases so obtained as streams offluid-from the downstreamedge of the fiameholder into the path travelled by; the main portion of the com bustion gases as they flow past and around the flameholder: This-increase ot'velocity can best be obtained by passing some or" the combustion gases through a passageway arranged toconverge inrthedownstream direction and to terminate in a narrow slot, an efiect which can readily be produced by the attachment of one or more vanes to the structure of the fiarneholder itself.

Figure 12 illustrates the theory of such modification by 'means of arsimple single line diagram representing a section through the V-gutterrof a flameholder. The gutter 3:0v has attached wit a. flatvane 31 which extendsalong such gutter in a generally parallel relationship therewith andis secured thereto by one or more struts 32.7,The combustion gases that enter the space definedbetween the gutter 30 and the vane 31 are constricted in the slot 33 that these, partstorm and, issue from. the downstream edge .of thefiameholder ata velocity greater thanthat of the .remainingacornbustion gases. that flow. past the outside of, the assembly and in eddiesw around behind it.

:These highvelocity streams of fluidv designated by, the arrowsA serve to disturb thenormal flow wconditions of .the-gasesin the jetpipe in. the vicinity of .the downstream wedge of thevflameliolder and thuslto-tbrealr up the .yortices ..that otherwise tend to appear in .this area and give,. rise direction for issuance of the streams.

to screeching combustion conditions. As i will ,appear fromv Fig ure 12, therhigh velocity gases, tend, byrre ason of the inclined face of the gutter 30, to issue from the downstream edge .at a slightly outwardly .inclined anglc, analogously with the inclination given to the spray' ztl in Figure 2. Such outward inclination of the streams of gases is not annessential. for alleviation of screeching combustion, but in most cases is. the most satisfactory Thetva'ne 31 is preferably. arranged to lie in a plane. paralleLwith the general direction of flow, so as to minimize its drag effect; but some inclination may be adopted, as is, in effect, what is done in the form of construction described 7 later with reference to Figures-l6 and 17.

,around part of the .vane itself. lt ist possible :to employ anti-screeching vanes along both edges of ,such a gutter,

but it has :heeniound by .experimentvthat asvane on one side-is normallysufficientin practice to. prevent the v initiation ofscreeching combustion.

t vFigure 7.14 illustratesra V.;ring flameholder 38, around theputer periphery .o f which anannularvane 39. ,is sup- V port'ed bywradialstruts .A main strut lcisvprovided it for the mounting ofthis ,flameholder in thejetpipe. In-

stead .of mounting. the anti-screeching vane, around; the

7 outside periphery of such .V -riug flameholder, it may be mounted around theinne-r periphery. Such asystem is illnstratedrin Figure l5which showsa downstream view ;;at 4 5.' .Practicalexperiments have,indicatedathatrrather better results may berexpected to be obtained-withthe igure 14; :arrangement:thanrzwith vthat" oflFigure 15; but

nevertheless: theaflameholder as modified .inuaccordance with Figure 15 provides significant reduction in the tendency of an engine to screech.

This would be an appropriate point to mention that the invention in its second aspect has been found to be not generally applicable to the simple cone type of flameholder illustrated in Figures 2 and 4. This is not because the use of vanes with a cone flameholder would not provide anti-screeching characteristics, but because the presence of a vane has been found normally to interfere with the basic flame-holding function of the flameholder. It would be necessary to modify the structure of a cone type flameholder in some other manner to overcome this difiiculty, if a vane were to be employed for anti-screeching purposes.

Measurements have been made of the additional drag on the engine represented by vanes such as those described above, and the increase of drag recorded was of the order of 15%. This is 15% of the total drag of the flameholder unmodified, which total drag is equivalent to only about 1% of the engine thrust. Thus no great reduction of thrust is caused by the use of vanes.

In a modified construction, no separate vane is provided. Instead, two portions of a flameholder are ar ranged sufliciently closely to one another so that each performs the function of an anti-screeching vane in relation to the other.

Figure 16 shows such an arrangement employed in a V-ring type of flameholder seen from the downstream side. This flameholder is composed of two concentric V-rings 46 and 47 mounted on a main strut 48 to define between them an annular slot 49. This modification also is applicable to the gutter type of flameholder and such an arrangement is illustrated in Figure 17 where a main strut 51 is shown supporting a series of parallel V-shaped gutters 50, again seen from the downstream side.

Experiments have been conducted with a view to determining the width of slot between flameholder and vane, or between two sections of a flameholder, that is most satisfactory in inhibiting screeching. A comparatively narrow slot is normally desirable as yielding the greatest increase of gas velocity; and this is normally convenient structurally, because it provides a compact construction and avoids the need for long struts. The minimum width found practical has been approximately inch. On the other hand, it has been found that the maximum width of slot that may be used may depend on the downstream width of the flameholder itself. This applies especially with wide flameholders, in which case the maximum slot width has been found to be of the order of half the downstream width of the flameholder.

For example, with a V-ring flameholder such as shown in Figure 14 and having a gutter width of four inches the best results were obtained with a slot Width between one and two inches. Above two inches, performance began to fall off. In another case, using a V-ring flameholder of only two inch gutter width, the best anti-screeching properties were obtained employing a slot between one and one and a half inches. On the other hand with a six-arm star type of V-gutter flameholder such as illustrated in Figure 13, excellent results were obtained with slots of one-quarter inch width.

It will be evident that the slot width that will provide best conditions must always remain to some degree a question to be decided by trial and error for each individual design of engine and flameholder. Nevertheless it may be said that the practical range from which the optimum slot width may be selected will extend from approximately inch to Whichever is the greater value between approximately 1 /2 inches and approximately half the downstream width of the flameholder. In the case where the flameholder is in two or more sections, the situation could arise that these sections (gutters) would be of different downstream widths. In this case it would be the downstream width of the wider gutter that would serve to determine the maximum slot width.

This application is a continuation-in-part of application Serial No. 523,402 filed July 21, 1955, now abandoned.

I claim:

In a jet engine of the type having a main combustion stage, a generally cylindrical jetpipe, and a reheat combustion stage comprising a flameholder mounted in said jetpipe and means for introducing reheat fuel upstream of said flameholder, said flameholder being of suflicient size by itself to maintain continuous combustion under all engine reheat operating conditions, means for inhibiting screeching combustion by disturbing and directing the normal flow conditions of gases at the downstream edge of said flameholder in a direction other than normal comprising a vane disposed adjacent said flameholder to extend in substantial parallelism to the longitudinal center line of said jetpipe, the downstream end of said vane terminating adjacent the downstream edge of said fiameholder whereby to form with said flameholder a passageway converging in the downstream direction and terminating in a narrow slot adjacent the downstream edge of said flameholder, said slot having a width lying Within the range from approximately /8 inch to whichever is the greater value between approximately 1 /2 inches and approximately half the downstream width of the flameholder.

References Cited in the file of this patent UNITED STATES PATENTS 2,607,191 Lee Aug. 19, 1952 2,622,396 Clarke et a1. Dec. 23, 1952 2,682,747 Ruegg July 6, 1954 2,693,083 Abbott Nov. 2, 1954 2,697,910 Brzozowski Dec. 28, 1954 2,714,287 Carr Aug. 2, 1955 2,720,754 Francois Oct. 18, 1955 2,771,743 Lovesey Nov. 27, 1956 2,780,916 Collins Feb. 12, 1957 FOREIGN PATENTS 165,939 Australia Nov. 8, 1955 

